The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for separating the signal contributions of two or more chemical species, such as water and fat, in MR images.
Iterative decomposition of water and fat with echo asymmetry and least squares estimation (“IDEAL”) is a multi-echo chemical species separation technique that has been shown to effectively separate water and fat with a number of different acquisitions. As described, for example, in U.S. Pat. No. 6,856,134, the IDEAL method employs a pulse sequence that acquires image data by sampling echo signals that occur at different echo times in a single repetition time (“TR”) period. An iterative, linear least squares approach, is used to estimate the separate water and fat signal components in images reconstructed from the acquired image data.
The IDEAL method was developed to address some of the limitations of conventional chemical species separation techniques, commonly termed Dixon methods, that exploit differences in resonance frequencies between chemical species. In particular, conventional Dixon methods require specific echo spacing, which can be difficult to reconcile with other timing requirements, such as a desired short pulse sequence TR. IDEAL allows for much greater flexibility in echo spacing and, thus, is more accommodating to the timing requirements of various sequences; however, IDEAL requires redundant sampling, which limits the spatial resolution achievable in a set scan time.
Previous attempts at improving the spatial resolution achievable with IDEAL have been generally unsatisfactory. For example, attempts have been made to use a balanced steady-state free precession (“bSSFP”) pulse sequence that samples multiple half-echoes along radial k-space trajectories; however, the spatial resolution achievable with this method was limited.
It would therefore be desirable to provide an imaging method in which high spatial resolution images can be acquired and used with chemical species separation techniques, such as IDEAL, to produce decomposed signals representative of the individual chemical species. For example, such a method would be beneficial for water-fat separation, in which high spatial resolution water and fat images could be produced.